FIG. 1 is an elevation view of a conventional single ended HID lamp 100 in accordance with the prior art. The HID lamp includes an arc tube 140 having at least two electrodes 120 and 130, a support wire 150, and a protective envelope 110 for housing the arc tube 140 and the support wire 150.
Examples of the HID lamp 100 include mercury vapor lamps, metal halide lamps, high and low pressure sodium vapor lamps, xenon short-arc lamps, and ultra-high performance mercury arc lamps. The HID lamp 100 produces light by generating an electric arc across two spaced-apart electrodes 120 and 130 housed inside the arc tube 140. The arc tube 140 can be a translucent or a transparent fused quartz or fused alumina arc tube. The electrodes 120 and 130 are typically fabricated using tungsten, but other materials can be used.
The arc tube 140 is suspended within an outer protective envelope 110 using the support wire 150. The support wire 150 is fabricated using an electrically conductive material that carries electricity to and from the arc tube 140. The support wire 150 also provides support for the arc tube 140 to be suspended within the outer protective envelope 110. The outer protective envelop 110 is fabricated using a transparent or translucent material that allows the light generated within the arc tube 140 to be emitted to a desired area for illumination.
The arc tube 140 also includes an arc tube cavity 141 having a top surface 144 and a bottom surface 146. The top surface 144 is the highest elevated portion of the arc tube cavity 141, while the bottom surface 146 is the lowest elevated portion of the arc tube cavity 141. The arc tube cavity 141 can be vertically oriented, horizontally oriented, or oriented at any angle therebetween. In FIG. 1, the arc tube cavity 141 is oriented vertically. Hence, the top surface 144 is located at the boundary between the electrode 120 and the arc tube cavity 141, and the bottom surface 146 is located at the boundary between the electrode 130 and the arc tube cavity 141. The arc tube cavity 141 has an arc tube diameter 142, which is the maximum width of the arc tube cavity 141.
The arc tube cavity 141 is typically filled with gas or a mixture of gas and metals. For example, the arc tube cavity 141 may be filled under pressure with pure xenon, a mixture of xenon-mercury, sodium-neon-argon, sodium-mercury-neon-argon, or some other mixture such as argon, mercury, and one or more metal halide salts. A metal halide salt is a compound of a metal and a halide, such as bromine, chlorine, or iodine. Some of the metals that have been used in metal halide lamps include scandium, sodium, thallium, and indium. Typically, xenon, neon, or argon gas is used in HID lamps because they are easily ionized and provide some level of light. These gases facilitate the striking of an arc between the two electrodes 120 and 130 when voltage is first applied to the HID lamp 100. Once the arc is started, the arc heats up and evaporates the metal salts thereby forming a plasma, which greatly increases the intensity of the light produced by the arc and reduces the power consumption.
The HID lamp 100 typically requires a ballast (not shown) to regulate the arc current flow and to deliver the proper voltage to the arc. Some HID lamps include a third electrode (not shown) within the arc tube that initiates the arc when the HID lamp is first lit. Alternatively, other HID lamps 100, such as the one shown in FIG. 1, use an igniter (not shown), or starting circuit, in lieu of the third electrode, to generate a high-voltage pulse to the electrodes 120 and 130 to start the arc. The formation of the arc requires a high current; but, once the arc is at steady-state conditions, much less current is required to operate the HID lamp 100. Compared with fluorescent and incandescent lamps, HID lamps 100 provide higher luminous efficacy since a greater portion of their radiation is in visible light as opposed to heat.
FIG. 2 is an illustration of a gravity effect 210 and a wire shadow 220 in an area 200 illuminated by the HID lamp 100 of FIG. 1 in accordance with the prior art. Typically, metal halide HID lamps exhibit the gravity effect 210 and the wire shadow 220; however, the gravity effect 210 and the wire shadow 220 can occur in other types of HID lamps. The gravity effect 210, or a yellow image, is an inherent color shift that settles in a particular spot in the arc tube 140. This color shift is based upon all of the components within the arc tube 140 and tends to slightly change the color and the intensity of the light that is emitted from the arc tube 140. According to typical HID luminaries (not shown), the optical system (not shown), which includes one or more reflectors and lenses, reflects the light emitted by the arc tube 140 to the area 200 that is illuminated, such as, a wall or a floor. The wire shadow 220 is a shadow of the support wire 150 that is formed in the area 200 that is illuminated. As previously mentioned, the support wire 150 is a wire or other mechanical means for suspending the arc tube 140 within the outer protective envelope 110 of the HID lamp 100. Although FIG. 2 illustrates the gravity effect 210 and the wire shadow 220 occurring substantially in the same location of the area 200 that is illuminated, the gravity effect 210 and the wire shadow 220 can occur in different locations.
In the past, some manufactures have attempted to minimize the wire shadow 220 by fabricating the support wire 220 using thinner and smaller wire sizes. Alternatively, manufactures have attempted to address both the gravity effect 210 and the wire shadow 220 by completely blocking the light emitting portion that includes the gravity effect 210 and the wire shadow 220 or by spreading out the entire light emission so that the gravity effect 210 and the wire shadow 220 are mixed with the rest of the emitted light. These solutions typically use a material having a highly diffusive finish on either or both of the reflector and the lens. The conventional lens is typically fabricated with prismatic elements formed across the entire surface of the lens. Alternatively or additionally, the reflector is typically fabricated as a pillow style reflector, which has numerous tiny bumps that are formed onto the entire reflector's inner surface. The use of a highly diffuse type of lens or a diffuse finish on the reflector's inner surface allows the light to be mixed and spreads the emitted light so that the gravity effect 210 and the wire shadow 220 is reduced or eliminated. However, the efficiency of the emitted light is substantially decreased when using these conventional solutions, because some of the generated light is reflected multiple times before being transmitted through the lens, while other portions of the generated light are never transmitted through the lens. Each time a ray of light bounces (or reflects) off the reflector's inner surface, the light emitting efficiency is reduced due to a loss of energy. In most conventional fixtures, approximately ten percent of the light's energy is absorbed each time the beam of light bounces off the reflector's inner surface. Thus, if the light bounces twice off the reflector's inner surface before being transmitted through the lens, the light efficiency is eighty-one percent, or (0.9)*(0.9)*(100%).
In view of the foregoing, there is a need in the art for providing a HID luminaire that reduces or eliminates the wire shadow and/or the gravity effect while improving lighting efficiency.